Both are fine in their own way.
Dell is good for business/student/casual use, while Asus is known for gaming use.
Dell's quality can depend on the computer honestly. Their higher-ends are known to have better quality. Asus is a bit the same way, but Asus really is known for gaming use.
If you are wanting to game, go Asus; if you just want a laptop for really just business/student/casual use, go Dell.